Cryptosporidium parvum has a complex life cycle with the alternation of asexual and sexual stages. Infection is initiated by ingestion of resistant sporulated oocysts; and merozoites are the only free forms of Cryptosporidium while trophozoites, meront and sexual stages are obligatorily intracellular stages. The Core Section of this Program Grant application will produce, purify and provide Cryptosporidium oocysts, sporozoites and intracellular stages to the investigators. With this purpose, new born calves will be experimentally infected, and oocysts and oocyst walls following excystation. Intracellular stages of Cryptosporidium will be obtained in Madin-Darby canine kidney (MDCK) cells, an epithelial cell line that is able to provide large amounts of intracellular parasites. The Core will coordinate the production of Cryptosporidium stages with the needs of the investigators for the projects detailed in this Program Grant application. The Core will be located at the Division of Infectious Diseases, Department of Medicine, San Francisco General Hospital.